From what I was told, your last period and the usual length of your cycles give the doctor a time frame in which you were most fertile and probably convieved, and your first due date will be based off of that. As you get further along and have a few ultrasounds (one at 8 weeks and another around 16-20) certain developments in the baby can show that you are further along than originally presumed, or you aren't as far as you thought.
My original due date was Sept. 12th based on LMP, but based on his development at 8 weeks it was changed to the 2nd. (that date always bugged me because it indicated that I concieved while I was on my period, during which I don't have sex... ) At my 20 week ultrasound my due date was moved again to the 7th. He was born on the 3rd.
Try not to worry about calculations with LMP, ultrasound is the best way to determine due date.
Good luck and congrats!
at 12:45 AM on Mar. 18, 2009